It is known to nitrate various aromatic compounds including, for example, phthalimides phthalic acids, phthalic anhydrides and derivatives thereof by, treatment with a mixture of concentrated sulfuric acid and nitric acid. Specific teachings of these nitration reactions may be found in Takekoshi, U.S. Pat. No. 3,868,389; Bacha et al, U.S. Pat. No. 4,137,419; and Cook et al, U.S. Pat. Nos. 3,933,852, 3,981,933 and 3,887,588.
Recently, it was discovered that aromatic compounds could be nitrated in a nitration reaction which employed only nitric acid. Specifically, copending patent application Ser. No. 917,926, filed Oct. 10, 1912, assigned to the same assignee as the present invention, discloses the nitration of alkylphthalimides in at least 95% concentrated nitric acid. Related copending patent application Ser. No. 886,172 filed May 22, 1986, also assigned to the same assignee as the present invention discloses the nitration of phthalic acids, phthalic anhydrides and derivatives thereof in a nitric acid only nitration process. Both of these copending patent applications teach the preferred use of high temperatures in order to achieve the best rate of nitration, thus short nitration times. However, even at such high temperatures, for example above 50.degree. C., the time for completion of the nitration, or substantially so, may be three hours or more.
More recently, copending patent application Serial No. 06/567,484 filed Jan. 3, 1984 disclosed that the rate of nitration in the nitric acid only nitration of aromatic substrates could be greatly enhanced by the addition of various acid anhydride nitration promoters to the reaction mixture. Specifically, the application disclosed that various organic and inorganic acid anhydrides were useful as nitration promoters in the nitric acid only nitration of aromatic substrates, particularly N-alkyl phthalimides, phthalic acids and phthalic anhydrides. That application further discloses that particular acid anhydride may be less effective or inoperable for certain aromatic substrates.
It has been now found that certain acid anhydride promoters which are generally effective promoters for the enhancement of the rate of nitration for various aromatic substrates are ineffective for the enhancement of the rate of nitration of phthalic acids, phthalic anhydrides and derivatives thereof, in the nitric acid only nitration process.
It is, therefore an object of the present invention to disclose effective acid anhydride nitration promoters for the nitric acid only nitration of phthalic acids, phthalic anhydrides and derivatives thereof.